Estimate time to finish a puzzle.
Puzzle time is piece count divided by placement rate, but a second person does not double throughput. Sorting the same pile and reaching across the same table create interference, modelled here as roughly a twelve percent efficiency loss per additional person. That is why a pair finishes a thousand-piece puzzle noticeably faster than one person but nowhere near twice as fast.
Effective team rate
Effective rate = individual rate x people x (1 - 0.12 x (people - 1)), floored at 40 percent efficiency
Time and sittings
Hours = pieces / effective rate / 60; sittings = ceiling(hours / hours per sitting)
Around one to three pieces a minute for an average adult on a thousand-piece image, faster during edges and distinctive regions, far slower on large areas of uniform colour or sky.
Because searching the pile is the bottleneck, not placing pieces. Two people searching the same pile duplicate effort unless they split by colour or region first.